A superconducting magnetic levitated train adopts a system in which a superconducting magnet is provided on the train and a normal conductive short-circuit coil on the ground side so that a current is applied to coil on the ground by electromagnetic induction caused when the magnetic flux of the superconducting magnet on the train cuts the coil on the ground at the time of starting the train, and the repulsive force between the superconducting magnet on the train and the coil on the ground levitates the train. The train is propelled by a linear synchronous motor system in which the thrust in the same direction is obtained by inverting the current of a propellant coil provided o the ground side separately from the normal conductive short-circuit coil by the interaction between the propellant coil and the superconducting magnet. Since such an induction current is utilized, no levitating force is obtained at the time of stop, and the levitation force is insufficient during low-speed travel, an auxiliary supporting device is required. However, levitation itself is naturally stable without the need for any control. In a superconducting magnet levitated train, since a strong magnetic field is obtained from the superconducting magnet on the train, the levitation height as large as about 100 mm can be realized. In addition, since the superconducting magnet is used in a permanent current mode, no power source for excitation is necessary on the train. On the other hand, a superconducting magnet suffers from a phenomenon of quenching in a normal conducting state, and a light-weight magnet having a high current density magnet such as a superconducting magnet for levitated trains involves a possibility of frequently quenching before the critical value of the superconducting wire is reached due to various disturbances. Since such a quench phenomenon is irreversible, once a quench phenomenon is produced, about 30 minutes is required for re-pouring a coolant to the superconducting magnet and exciting the magnet.
In a conventional superconducting magnetic levitated train such as shown in, for example, FIGS. 2 and 3 two superconducting magnets 2 are accommodated in one coolant container 3 and four coolant containers (eight superconducting magnets) are attached to a car truck 10 provided at the lower part of a car body 1 in each car. An auxiliary supporting device 7 supports the train at the time of stop and during a low-speed travel, while at a high-speed travel, the train is levitated by the repulsive force between a propellant guiding ground coil 4B, a supporting ground coil 4A and the superconducting magnets 2 on the train. Additionally, it is proposed that in a line for traffic experiment or in a traffic train in the near future, the car trucks 10 carrying the superconducting magnets 2 are gathered between the cars in order to reduce the magnetic field leaking from the superconducting magnets 2 to a passenger car.
In such a superconducting magnetic levitated train, when a specific superconducting magnet quenched during travel, the superconducting magnet situated at the symmetrical position is forced to be demagnetized so as to prevent the balance of the train as a whole from being lost by the levitation force at the part of the quenched superconducting magnet and the train is cause to safely travel and landed by an emergency landing device 8, as described in Japanese Patent Publication U.S. Pat. No. 1,376,957. U.S. Pat. No. 881,534 discloses a connecting system for preventing the loss of the balance on the left-hand and right-hand sides by modifying the arrangement of the group of superconducting magnets so that the S-pole, N-pole and S-pole are alternately disposed in the longitudinal direction of the train and the same poles are connected in series to each other.
For a superconducting wire in a conventional magnetic levitated train, multifilamentary NbTi wire having a low copper ratio is used in order to reduce the weight of a superconducting magnet and increase the current density of a superconducting coil, as described on 36 to 40 pages of "The Journal of the Japan Society of Mechanical Engineering", vol. 91, No. 835, June (1988). A Cu/NbTi wire containing 1.0 of Cu based on NbTi in cross sectional ratio (hereinunder referred to as "copper ratio") is known. Although a wire using aluminum in place of Cu or multifilamentary NbTi wire using aluminum in place of a part of Cu is proposed in order to reduce the weight, since such wire has a large aluminum cross sectional area, it is not used for a superconducting magnetic levitated train.
In any of the above-described prior art, the facts that when a specific superconducting magnet on the train quenches, the superconducting magnetic levitated train during high-speed travel must be suddenly stopped and that a long time is required for restoration are not taken into any consideration. Therefore, such a conventional superconducting magnetic levitated train is lacking in the reliability as a traffic train.
In addition, a superconducting coil composed of a wound multifilamentary NbTi wire having a low copper ratio, especially, a wire having a copper ratio of 1.0 is poor in the electromagnetic stability. If a magnetic levitated train with such a superconducting coil mounted thereon travels at a high speed, a superconducting coil on the train frequently quenches due to the increase in disturbance energy. In a wire using aluminum in place of Cu or a wire using aluminum in place of a part of Cu, a coil having a high current density is taken into almost no consideration, so that if it is mounted on a superconducting magnetic levitated train, the levitation force is inconveniently small.
Accordingly, it is an object of the present invention to eliminate the defects in the prior art and to provide a superconducting magnetic levitated train, a superconducting magnetic levitated train system and a method of controlling the same, and a superconducting coil for a magnetic levitated train which are unlikely to cause quench even during high-speed travel and which have a high reliability.
It is another object of the present invention to eliminate the defects in the prior art and to provide a highly reliable method of controlling a superconducting magnetic levitated train which controls a superconducting coil before it quenches by varying the stability margin of a superconducting magnet and the magnitude of a disturbance so as to prevent a quench phenomenon even during high-speed levitation.
To achieve this aim, in the present invention, the stability margin of a superconducting magnet on a specific car in which the disturbance energy is large on the basis of the fact that the disturbance energy applied to a superconducting magnet is different in cars.
The present invention provides a superconducting magnetic levitated train having a plurality of cars connected to each other, a group of superconducting magnets attached to the car trucks and a coil provided on the ground so that the superconducting magnetic levitated train is levitated by the magnetically induced repulsion between the superconducting magnets and the coil on the ground, characterized in that a superconducting magnet on a specific car has a predetermined stability margin which is larger than the stability margin of a superconducting magnet on the other cars.
The present invention also provides a method of controlling a superconducting magnetic levitated train having a plurality of cars connected to each other, a group of superconducting magnets attached to the car trucks and a coil provided on the ground so that the superconducting magnetic levitated train is levitated by the magnetically introduced repulsion between the superconducting magnets and the coil on the ground, the method comprising the steps of: detecting the driven state of a superconducting magnet on a specific car; varying and controlling the travelling conditions for the train so that the stability margin of the superconducting magnet on the specific car is changed in accordance with the command from a train control means by varying the current value in a permanent current mode by an auxiliary power source and a resistor; and making the stability of the margin of the superconducting magnet on the specific car larger than the stability margin of a superconducting magnet on the other cars by varying the stability margin of the superconducting magnet on at least one car.
In the present invention, it is possible to make the stability margins of the superconducting magnets on the lead car and the rearmost car larger than the stability margins of the superconducting magnets on the other cars therebetween. The present invention is more effective when the stability margin of a superconducting magnet mounted on a car is not less than 200 mJ/cc in rated drive.
Appropriate control carried out so that the stationary disturbance energy applied to a superconducting magnet on a car is restricted to not more than 100 mJ/cc is effective.
In the present invention, use of a superconducting coil composed of a wound composite superconducting wire which is stabilized by aluminum is effective. The composite superconducting wire which is stabilized by aluminum is multifilamentary NbTi wire having a low copper ratio with the outer surface thereof coated with thin high-purity aluminum or multifilamentary NbTi wire having a low copper ratio accommodating high-purity aluminum at the central part of the cross section thereof.
According to the present invention, it is possible that the stability margin is variable in correspondence with the disturbance energy which is different depending upon the position of the car. Furthermore, in a superconducting magnet on a specific car, a detector which is capable of detecting the magnitude of a disturbance, an auxiliary current source which is capable of varying the current value in a permanent current mode and a resistor are arranged, so that a centralized train control center provided on the ground can control the stability margins of the superconducting magnets or the speed of the train.
The "stability margin" in the present invention means the difference between a critical characteristic (temperature, magnetic field, current density) of a superconducting magnet and the operating point obtained when the magnet is actually used. The stability margin represents at least one parameter of the temperature, magnetic field and current density of the magnet by an energy per unit volume of the magnet (mJ/cc).
In contrast, a factor in the reduction of the travelling stability of a magnetic levitated train is a disturbance energy. Causes for a disturbance energy are the proximity of the train to a tunnel, the existence of a high-voltage steel tower and the like, and the disturbance energy produced by such a cause can be obtained by calculation at the time of designing the magnetic levitated train system. As a result of the investigation of the present inventor, it has been found that the stability margin of a superconducting magnet on a specific car which is required to enhance the stability must be so designed as to be constantly larger than the disturbance energy and that the effective difference between the former and the latter is not less than 50 mJ/cc, preferably not less than 100 mJ/cc.
If the superconducting magnet, the train and the system are so designed that the stability margin of the superconducting magnet is not less than 200 mJ/cc and the magnitude of the disturbance energy is not more than 100 mJ/cc, especially stable levitation is possible.
Generally, in order that a superconducting coil is stable at a certain operating point, it is necessary that the stability margin which the superconducting coil has is larger than the heat energy produced by the disturbance due to an electromagnetic energy or a mechanical energy, with respect to a superconducting magnetic levitated train, the stability margin and the heat energy produced by a disturbance were quantitatively calculated by experiments.
The superconducting coil for magnetic levitated train does not contain a helium cooling channel, in other words, it is what is called a closely wound coil. An epoxy resin is inserted between the turns of the winding for fixture. The winding is accommodated in a liquid helium tank which is called an inner tank, and a spacer is intermittently inserted between the winding and the inner tank so that the liquid helium exists in a space which corresponds to the thickness of the spacer.
It was experimentally made clear that the stability margin in such a coil structure allows a larger value than the heat capacity which is calculated by using an adiabatic stabilization model and which is determined by the temperature margin and the heat capacity of the superconducting wire. In other words, it is necessary to consider a dynamic stabilization model including the sectional area, the electric resistance and the thermal conductivity of a stabilizing material. Therefore, if the following improvements are adopted, the stability margin of a superconducting coil for a magnetic levitated train is increased.
(i) The current load factor to the superconducting wire of the superconducting coil is reduced (temperature margin is increased).
(ii) A stabilizing material having a large heat capacity at cryogenic temperature is used.
(iii) The sectional area of the stabilizing material is increased.
(iv) The electric resistance of the stabilizing material at cryogenic temperature is reduced.
(v) A stabilizing material having a high thermal conductivity at cryogenic temperature is used.
A material which satisfies the above-described conditions, which is light in weight and the current density of which can be enhanced as a composite superconducting wire which is stabilized aluminum. However, since aluminum is mechanically so soft that it is impossible to produce multifilamentary NbTi wire having aluminum for the whole matrix in the present wire working technique. It has been found that it is possible to greatly increase the stability margin by using a superconducting wire having Cu and aluminum for a composite matrix, wherein a slight amount of aluminum which enables plastic working of such a composite matrix wire suffices.
In a superconducting magnetic levitated train which travels at a super high speed and which has a plurality of connected cars, the kind and the magnitude of the disturbance applied to a superconducting magnet on the train will be inferred. As a disturbance applied to a general superconducting magnet, there are a movement of the superconducting wire, cracking of the epoxy resin between the turns of the windings and the deformation of the superconducting coil as a whole which are caused by an electromagnetic force produced by the excitation of a superconducting coil and which generate heat which resulting in the production of quench at not more than a rated value. These disturbances, however, can be detected by test before mounting the coil on the train as a problem at the time of winding. As the disturbances characteristic of a superconducting magnetic levitated train, generation of the alternating current loss produced on the superconducting coil by the fluctuating magnetic field from the coil on the ground, the increase in the heat ingress due to vibration and the generation of heat due to the impact load, vibration and wind pressure produced when the train enters a tunnel or the train passes another train will be considered. It is considerably difficult to quantify the magnitudes of these disturbance energies in the present state of art. It is, however, certain that the disturbance energy applied to a superconducting magnet is different depending upon the position of the car in a superconducting magnetic levitated train having a plurality of connected cars. This fact is one of the basis of the present invention. That is, a superconducting magnet on the lead car receives the largest disturbance energy, and it is the lead car and the rearmost car which are susceptible to the influence of the lowered levitation force at the time of quench. In the present invention, the car which is liable to receive a disturbance is specified and the stability margin of a superconducting magnet on the specified car is made larger than the stability margin of a superconducting magnet on other cars. In the present invention, a car which is susceptible to the influence of the lowered levitation force at the time of quench is discriminated and the stability margin of a superconducting magnet on the specified car is made larger than the stability margin of a superconducting magnet on other cars.
The concrete stability margin was calculated by experiment. Four kinds of superconducting wires having stabilizing materials having different sectional areas and different materials were respectively wound so as to have a cross section similar that of a superconducting coil for a magnetic levitated train and an epoxy resin was inserted between the turns of winding, thereby producing superconducting coils. In order to simulate a disturbance energy, a pulsating current was applied to a heater wire embedded in the winding in advance, thereby generating a thermal disturbance. In this way, the minimum energy necessary for quench was obtained by experiment. As a result, the stability margin of the coil of a conventional multifilamentary NbTi wire having a copper ratio of 1.0 was 100 mJ/cc at the rated value of the coil and it was found that the stationary disturbance energy (except a disturbance due to a non-stationary disaster such as earthquake, fire and explosion) applied to a superconducting magnet must be not more than 100 mJ/cc according to the results of the current test operations of a superconducting magnetic levitated train. From the results of the experiments on the stability margins of the coils of wires having different copper ratios, it was made clear that a stability margins of not less than 200 mJ/cc is necessary in order to secure the stable margin of a superconducting magnet on the train with a high reliability.
As is clear from the above explanation, the disturbance energy on a superconducting magnetic levitated train varies with the speed of the train, the position of the car and the environmental conditions of the travelling train. Another feature of the present invention is that the stability margin of each superconducting magnet is made variable in correspondence with temporally and spatially different disturbance energies. With respect to a spatially different disturbance energy, superconducting magnets having different stability margins are mounted at predetermined positions in advance. With respect to temporally different disturbance energies, it is necessary to detect the magnitude of the disturbance during high-speed travel and control the stability margins of the superconducting magnets on the train or the speed of the train. The control of the speed of the train is easily carried out by varying the frequency of the current which is applied to the propellant coil on the ground. For example, a method of reducing the disturbance energy applied to the superconducting magnets by reducing the speed of the train immediately before the train enters a tunnel or the train passes another train is effective. Furthermore, it is possible to control the coil current on the train in a permanent current mode by a centralized train control center provided on the ground.
By virtue of the features of the present invention, it is possible to safely drive the superconducting magnetic levitated train at a super high speed with the greatest efficiency by specifying a car which is susceptible to a disturbance and increasing the stability margin of the superconducting magnet mounted on the specified car.
The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show, for the purpose of illustration only, several embodiments in accordance with the present invention.